1. Field of the Invention
The present invention relates to a technique for improving the crystal quality of a group-III nitride crystal
2. Description of the Background Art
A group-III nitride crystal has been used as a material constituting a semiconductor device such as a photonic device and an electronic device, and has recently received attention as a semiconductor material constituting a rapid IC chip for use in a portable telephone. Attention is particularly being given to an AlN film as a material for application to a field emitter.
For such device applications, a group-III nitride crystal is ideally provided as free-standing one. Under the current circumstances, however, a group-III nitride crystal is typically provided in the form of a so-called epitaxial substrate epitaxially formed on a predetermined single crystal base considering problems with crystal quality, manufacturing costs and the like. In general, thin film formation methods such as an MOCVD (metal-organic chemical vapor deposition) process and an MBE (molecular beam epitaxy) process are used to form such an epitaxial substrate.
It is necessary to improve the crystal quality as much as possible in order to increase the performance of a functional device when a group-III nitride crystal is intended to function as the functional device. For instance, when an epitaxial substrate is used for a functional device, it is typical that a single crystal layer is further formed on the epitaxial substrate. In order to provide the single crystal layer with a good crystal quality, it is necessary to minimize the dislocation density in the surface of a group-III nitride crystal constituting the epitaxial substrate. The reduction in dislocation density promises to achieve, for example, an increase in light emitting efficiency for a light-emitting device, a decrease in dark current for a photodetector device, and an increase in mobility for an electronic device.
In such an epitaxial substrate, however, a lattice mismatch between the base and the group-III nitride crystal to give rise to dislocations at an interface therebetween. Such dislocations thread through into the surface of the group-III nitride crystal. Accordingly, the crystal quality of the obtained group-III nitride crystal is not necessarily sufficient. For application to a functional device, an improvement in such crystal quality is required, and therefore, various methods have been proposed so far.
Particularly for achieving a light receiving/emitting device of short wavelength, it is important to form a group-III nitride film containing Al and having a low dislocation density. In this connection, there is a method by which a group-III nitride film at least containing Al and AlN are grown at high temperatures to be used as an underlying layer (e.g., Japanese Patent Application Laid-Open No. 9-64477 (1997)). This JP 9-64477 further discloses a technique for annealing a buffer layer at high temperatures to remove strains.
In addition, a method is proposed by which a group-III nitride film at least containing Al is grown at high temperatures with irregularities to be used as an underlying layer (Japanese Patent Application Laid-Open No. 2002-222771).
The technique disclosed in JP 9-64477 can form a group-III nitride film containing Al, but does not reduce the dislocation density to a sufficient degree. The technique disclosed in JP 2002-222771 can also form a group-III nitride film containing Al, but the use of this technique does not reduce the dislocation density in an underlying layer to a sufficient degree. Further reduction in dislocation density has therefore been required.